D/A converters are today used in a wide range of applications. In such applications, a quantized analog signal must always be produced from a digital signal using a quantization device. The usual problem in this context is that the quantization device, which often comprises a multiplicity of quantization elements, cannot ensure an arbitrarily high level of accuracy for the quantized analog output signal.
To overcome the problem of inaccurate or imprecise quantization elements in D/A converters, it is known practice to use DEM (Dynamic Element Matching), as described in “Design of Multibit Delta-Sigma A/D converters” by Yves Geerts, Michael Steyaert, Willy Sansen, Kluwer Academic Publisher, ISBN 1-4020-7078-0, on pages 74 to 97. A drawback of using DEM is that D/A converters with high resolution, i.e. with a large number of quantization elements in the quantization device, require very complex hardware for this.
High-resolution D/A converters are therefore known to be preferably provided as an array arrangement of current sources, as described in European patent specification EP 0 176 981. FIG. 5 shows an example of how the current sources for the individual cells 23 in the array arrangement 22 are connected in a conventional D/A converter, formed from a current source array arrangement 22. In this case, a predetermined number of single cells 24 is activated, i.e. the current sources in the single cells are turned on, starting from a first cell in the top left-hand corner of the array arrangement 22 in line with a digital input signal. The individual currents from the current sources of the single cells are added at the output to form a current signal at a corresponding level. The level can essentially be calculated from the number of activated single cells times the current value for each single cell, which is assumed to be the same.
First, the drawback arises that the cells 23 in the initial region, starting at the first cell at the top left, are used very much more or more frequently than the cells 23 in the final region, particularly the last cell 23 in the array arrangement 22 at the bottom left. Another drawback is that each single cell 23 in practice does not deliver an exactly identical output current, such as the adjacent activated cell 24. As a result, a quantization error arises which corrupts or distorts the quantized analog current output signal from the array arrangement 22. The quantization error for the initial cells 23, starting at the first cell at the top left, is then included in the output signal again and again.
It is therefore an object of the present invention to provide a D/A converter which generates a small quantization error at a high resolution. The invention achieves this object by means of a digital-analog converter and by means of a method for digital-analog conversion in accordance with embodiments of the invention.
The idea on which the present invention is based essentially involves combining a DEM device with a high-resolution D/A converter which has an array arrangement comprising cells, preferably with current sources. This allows the area or a cohesive block of the energy sources, preferably current sources, which are turned on to be connected to any point in the array arrangement. It is thus possible for the cells in the array arrangement and hence the individual, normally imprecise, energy sources to be interchanged dynamically. In addition, each energy source, preferably current source, for the cells in the array arrangement will accordingly be turned on with the same frequency, as a result of which essentially a random spread of the individual quantization errors of a single cell is achieved in the influence on the quantized analog output signal.
The present invention solves the problem cited at the outset particularly by providing a D/A converter having: a DEM logic device for generating at least two digital output data items from the digital input data on the basis of a predetermined algorithm to determine an initial cell and a final cell in the array arrangement, between which there are situated cells with energy sources to be activated; a decoder device for decoding the at least two digital output data items from the DEM device into actuation signals in order to activate the cells which are to be activated; and an array arrangement of cells for outputting at least one quantized analog signal on the basis of the actuation signals.
In line with one preferred development, the array arrangement has single cells with a respective current source.
In line with a further preferred development, the DEM logic device has a parallel input for supplying the digital input data, which have a predetermined bit length.
In line with a further preferred development, the output of the DEM logic device has two digital output data items, an arithmetic sign signal and a clock signal which are coupled to the decoder device.
In line with a further preferred development, the output of the decoder device has two row actuation signals and three column actuation signals and preferably two associated complementary row actuation signals and three complementary column actuation signals which are coupled to the array arrangement for the purpose of activating energy sources for predetermined cells.
In line with a further preferred development, the array arrangement has two mutually inverse quantized analog output signals.
In line with a further preferred development, the array arrangement has single cells with a respective local decoder device whose input respectively has two row actuation signals and three column actuation signals and preferably two associated complementary row actuation signals and three complementary column actuation signals.
In line with a further preferred development, the array arrangement has a respective edge length of at least 64 cells, corresponding to a bit length for the input signal of at least 12 bits.
In line with a further preferred development, an initial cell and a final cell in the array arrangement, between which there are situated cells with activated energy sources, are determined in the DEM device from the digital input data on the basis of a predetermined algorithm, and particularly when the activated cells reach the last cell in the array arrangement cells are activated in a manner adjoining the first cell in the array arrangement.
In line with a further preferred development, a DWA (Data Weighted Averaging) algorithm or a bi-DWA (bidirectional Data Weighted Averaging) algorithm or an ILA (Individual Level Averaging) algorithm is used in the DEM device in order to determine the cells in the array arrangement which are to be activated.
In line with a further preferred development, a local decoder device in a cell in the array arrangement connects an energy source in the cell to an output of the decoder device when a first column signal and a first row signal, or a second column signal and a second row signal, or a third column signal are activated.